It's the grain. He can't handle all the sugar, 8% and starch, 20%. NSC should be more like half that. All that starch will give him lots of problems including hind gut acidosis/ulcers. "Causes of Subclinical Acidosis Certain situations trigger the pH of the hindgut to drop sharply. The two most common causes are the overconsumption of high-starch concentrates or pasture grasses rich in fructan. The demands placed on horses—as athletes and as breeding animals— dictate that substantial quantities of energy-laden feeds be consumed." Subclinical Acidosis: Is Your Horse at Risk? - Kentucky Equine Research Since Nutrena won't do a fixed feed, it is probably a lot of corn and maybe oats to get that high of starch. My horse did that on Safe Choice Original last year, sugar 5% and starch 17%, all corn, and it was lower than this grain. Also gave him hind gut acidosis and put him over the edge into insulin resistance. He now eats Triple Crown Senior, 5.3% sugar and 6.4% starch, zero corn. Before he wasn't big into grain especially with any supplements in it. Now he has no issues with me putting supplements in his grain, even powdered magnesium. Before and after shots to show before, at his worst and started improving. And him May 30, even more improved.